1GRDFFT(1) Generic Mapping Tools GRDFFT(1)
2
6 grdfft - Perform mathematical operations on grid files in the wavenum‐
7 ber (or frequency) domain
8
10 grdfft in_grdfile -Gout_grdfile [ -Aazimuth ] [ -Czlevel ] [
11 -D[scale|g] ] [ -E[x|y][w] ] [ -F[x|y]params ] [ -I[scale|g] ] [ -L ] [
12 -M ] [ -Nstuff ] [ -Sscale ] [ -Tte/rl/rm/rw/ri ] [ -V ]
13
15 grdfft will take the 2-D forward Fast Fourier Transform and perform one
16 or more mathematical operations in the frequency domain before trans‐
17 forming back to the space domain. An option is provided to scale the
18 data before writing the new values to an output file. The horizontal
19 dimensions of the grid are assumed to be in meters. Geographical grids
20 may be used by specifying the -M option that scales degrees to meters.
21 If you have grids with dimensions in km, you could change this to
22 meters using grdedit or scale the output with grdmath.
23 in_grdfile
25 2-D binary grid file to be operated on. (See GRID FILE FORMATS
26 below).
27 -G Specify the name of the output grid file. (See GRID FILE FOR‐
29 MATS below).
30
32 No space between the option flag and the associated arguments.
33 -A Take the directional derivative in the azimuth direction mea‐
35 sured in degrees CW from north.
36 -C Upward (for zlevel > 0) or downward (for zlevel < 0) continue
38 the field zlevel meters.
39 -D Differentiate the field, i.e., take d(field)/dz. This is equiv‐
41 alent to multiplying by kr in the frequency domain (kr is radial
42 wave number). Append a scale to multiply by (kr * scale)
43 instead. Alternatively, append g to indicate that your data are
44 geoid heights in meters and output should be gravity anomalies
45 in mGal. [Default is no scale].
46 -E Estimate power spectrum in the radial direction. Place x or y
48 immediately after -E to compute the spectrum in the x or y
49 direction instead. No grid file is created; f (i.e., frequency
50 or wave number), power[f], and 1 standard deviation in power[f]
51 are written to stdout. Append w to write wavelength instead of
52 frequency.
53 -F Filter the data. Place x or y immediately after -F to filter x
55 or y direction only; default is isotropic. Choose between a
56 cosine-tapered band-pass or a Gaussian band-pass filter.
57 Cosine-taper: Specify four wavelengths in correct units (see -M)
58 to design a bandpass filter: wavelengths greater than lc or less
59 than hc will be cut, wavelengths greater than lp and less than
60 hp will be passed, and wavelengths in between will be cosine-
61 tapered. E.g., -F1000000/250000/50000/10000 -M will bandpass,
62 cutting wavelengths > 1000 km and < 10 km, passing wavelengths
63 between 250 km and 50 km. To make a highpass or lowpass filter,
64 give hyphens (-) for hp/hc or lc/lp. E.g., -Fx-/-/50/10 will
65 lowpass x, passing wavelengths > 50 and rejecting wavelengths <
66 10. -Fy1000/250/-/- will highpass y, passing wavelengths < 250
67 and rejecting wavelengths > 1000. Gaussian band-pass: Append
68 two wavelengths in correct units (see -M) to design a bandpass
69 filter. At the given wavelengths the Gaussian filter weights
70 will be 0.5. To make a highpass or lowpass filter, give a hyphen
71 (-) for the hi or lo wavelength, respectively. E.g., -F-/30
72 will lowpass the data using a Gaussian filter with half-weight
73 at 30, while -F400/- will highpass the data.
74 -I Integrate the field, i.e., compute integral_over_z (field * dz).
76 This is equivalent to divide by kr in the frequency domain (kr
77 is radial wave number). Append a scale to divide by (kr *
78 scale) instead. Alternatively, append g to indicate that your
79 data set is gravity anomalies in mGal and output should be geoid
80 heights in meters. [Default is no scale].
81 -L Leave trend alone. By default, a linear trend will be removed
83 prior to the transform.
84 -M Map units. Choose this option if your grid file is a geographi‐
86 cal grid and you want to convert degrees into meters. If the
87 data are close to either pole, you should consider projecting
88 the grid file onto a rectangular coordinate system using grdpro‐
89 ject.
90 -N Choose or inquire about suitable grid dimensions for FFT. -Nf
92 will force the FFT to use the dimensions of the data. -Nq will
93 inQuire about more suitable dimensions. -Nnx/ny will do FFT on
94 array size nx/ny (Must be >= grid file size). Default chooses
95 dimensions >= data which optimize speed, accuracy of FFT. If
96 FFT dimensions > grid file dimensions, data are extended and
97 tapered to zero.
98 -S Multiply each element by scale in the space domain (after the
100 frequency domain operations). [Default is 1.0].
101 -T Compute the isostatic compensation from the topography load
103 (input grid file) on an elastic plate of thickness te. Also
104 append densities for load, mantle, water, and infill in SI
105 units. If te == 0 then the Airy response is returned. -T
106 implicitly sets -L.
107 -V Selects verbose mode, which will send progress reports to stderr
109 [Default runs "silently"].
110
112 By default GMT writes out grid as single precision floats in a COARDS-
113 complaint netCDF file format. However, GMT is able to produce grid
114 files in many other commonly used grid file formats and also facili‐
115 tates so called "packing" of grids, writing out floating point data as
116 2- or 4-byte integers. To specify the precision, scale and offset, the
117 user should add the suffix =id[/scale/offset[/nan]], where id is a two-
118 letter identifier of the grid type and precision, and scale and offset
119 are optional scale factor and offset to be applied to all grid values,
120 and nan is the value used to indicate missing data. When reading
121 grids, the format is generally automatically recognized. If not, the
122 same suffix can be added to input grid file names. See grdreformat(1)
123 and Section 4.17 of the GMT Technical Reference and Cookbook for more
124 information.
125 When reading a netCDF file that contains multiple grids, GMT will read,
127 by default, the first 2-dimensional grid that can find in that file. To
128 coax GMT into reading another multi-dimensional variable in the grid
129 file, append ?varname to the file name, where varname is the name of
130 the variable. Note that you may need to escape the special meaning of ?
131 in your shell program by putting a backslash in front of it, or by
132 placing the filename and suffix between quotes or double quotes. The
133 ?varname suffix can also be used for output grids to specify a variable
134 name different from the default: "z". See grdreformat(1) and Section
135 4.18 of the GMT Technical Reference and Cookbook for more information,
136 particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
137
139 To upward continue the sea-level magnetic anomalies in the file
140 mag_0.grd to a level 800 m above sealevel:
141 grdfft mag_0.grd -C800 -V -Gmag_800.grd
143 To transform geoid heights in m (geoid.grd) on a geographical grid to
145 free-air gravity anomalies in mGal:
146 grdfft geoid.grd -Dg -M -V -Ggrav.grd
148 To transform gravity anomalies in mGal (faa.grd) to deflections of the
150 vertical (in micro-radians) in the 038 direction, we must first inte‐
151 grate gravity to get geoid, then take the directional derivative, and
152 finally scale radians to micro-radians:
153 grdfft faa.grd -Ig -A38 -S1e6 -V -Gdefl_38.grd
155 Second vertical derivatives of gravity anomalies are related to the
157 curvature of the field. We can compute these as mGal/m^2 by differen‐
158 tiating twice:
159 grdfft gravity.grd -D -D -V -Ggrav_2nd_derivative.grd
161 The first order gravity anomaly (in mGal) due to the compensating sur‐
163 face caused by the topography load topo.grd (in m) on a 20 km thick
164 elastic plate, assumed to be 4 km beneath the observation level can be
165 computed as
166 grdfft topo.grd -T20000/2800/3330/1030/2300 -S0.022 -C4000
168 -Gcomp_faa.grd
169 where 0.022 is the scale needed for the first term in Parker's expan‐
171 sion for computing gravity from topography (= 2 * PI * G * (rhom -
172 rhol)).
173
175 GMT(1), grdedit(1), grdmath(1), grdproject(1)
176GMT 4.3.1 15 May 2008 GRDFFT(1)